This application is a 371 of PCT/GB92/00822 filed on May 6, 1992.
The present invention relates to a battery of high temperature secondary cells in a battery casing having means enabling multiple electrical connections from outside the casing, in particular, a battery of the sodium sulphur type.
The sodium sulphur type battery is a form of secondary battery employing molten sodium and sulphur as the electrode materials. The potential advantages of the sodium sulphur battery over, for example, lead acid batteries are its lightweight, high storage capacity and relatively quick recharging time. Furthermore, sodium and sulphur are both materials which are cheap and abundant. A typical sodium sulphur battery is described in International patent application No. PCT/GB88/00500 (WO 89/00344) which is in the name of the applicant.
In order to provide a battery capable of powering a vehicle, it may be necessary to assemble as many as 3000 individual cells in arrays. The cells are first connected in series to form strings, the strings are then connected in parallel to form banks and the banks themselves are then connected in series. Whem sodium sulphur cells are fully charged they have a high resistance. Thus, it is necessary to provide separate connections to each of the series connected banks, so that a fully charged bank can be by-passed to ensure that remaining banks can themselves be fully charged. Sodium sulphur batteries must operate at temperatures of about 350.degree. C. and are housed in thermally insulating casings to prevent excessive heat loss. Connections to the various banks of cells within the battery casing must be capable of withstanding such high operating temperatures without degradation of electrical insulation. Further, in order to provide charging current selectively to individual banks of cells within the battery, the connections to the banks must be taken through the thermally insulating battery casing for connection to external charging apparatus. These lead outs through the battery casing should be designed to avoid excessive heat loss through the casing.
Whilst the need for multiple lead-outs from each series connected bank of a battery has been described above for sodium sulphur batteries, similar requirements may arise for other kinds of battery formed of high temperature cells. Generally, for the purpose of this specification, high temperature batteries and high temperature cells are defined as those required to be at above 150.degree. C. in order to operate.
As mentioned earlier, a problem encountered when there are numerous lead outs is that the lead outs not only serve their intended electrical function but also provide a good path for heat to be conducted out of the battery. Steps must, therefore, be taken to minimize heat loss which occurs in this way and in addition, they must be capable of withstanding the flow of high current.